Dealing with Galaxy 15: Zombiesats and on-orbit servicing
by Brian Weeden
|The inability to easily and accurately determine what caused Galaxy 15’s malfunction is a strong incentive to improve the ability to attribute on-orbit failures, both to try and create solutions and to reduce tensions that could arise from a case of assumed hostile action.|
Over the last two months that this story has developed, many significant questions have been raised about the long-term viability of operations in the GEO region given the current operational practices of global military, civil, and commercial operators. This article examines the Galaxy 15 event in detail, explains what happened and why it poses such serious problems, and discusses some recommendations for dealing with this in the future in such a way we can continue to use space for benefits on Earth.
Part one of this article will discuss the Galaxy 15 situation in detail, based on all the facts and reporting to date. Part two will examine the orbital mechanics of the GEO orbit and the implications for situations like Galaxy 15. Part three looks at the radiofrequency interference concerns. Part four discusses solutions to future situations of this nature and presents policy recommendations. In summary:
According to news reports, on April 5, Galaxy 15 stopped responding to commands from ground operators. Galaxy 15 was providing a variety of media services to North American customers, including video transmissions, and also had a payload used by the US Federal Aviation Administration. Intelsat quickly decided to move one of its on-orbit spare satellites, Galaxy 12, from a holding location to take Galaxy 15’s spot and customers. Since the satellite continued to provide service to customers, originally Intelsat deemed the anomaly not terribly serious. It would take a while for Galaxy 15 to drift far enough where its service was disrupted; by then Galaxy 12 would be in place and able to take over.
On April 20, Orbital Sciences, the company which built Galaxy 15, suggested that the communications problems with Galaxy 15 were potentially caused by a large geomagnetic storm occurring in space. In the early morning hours of April 5 the NOAA Space Weather Prediction Center in Colorado released a space weather advisory warning bulletin about the storm. Galaxy 15 had come out of the Earth’s shadow and into view of the Sun as this storm was occurring, and some experts suspect this event somehow damaged the satellite’s ability to receive or execute commands. However, this has not nor may never be fully verified, in large part because of the lack of ability to correlate space weather with specific malfunctions and failures.
Whatever malfunction did occur did not affect either the satellite’s ability to re-broadcast signals or its ability to keep its transponders pointed at the Earth and solar panels aligned with the Sun (known as “Earth lock”). This allowed the spacecraft to continue to receive and transmit signals. What it did affect was the ability of Intelsat’s ground controllers to maneuver Galaxy 15 to maintain its orbital position. Intelsat issued between 150,000 and 200,000 commands to the satellite in an attempt to get a response to either turn off its communications payload or maneuver. When these efforts failed, the company attempted to send an even stronger signal to try and force an overload of the satellite’s power system and cause it to shut down. This too failed. As a result, the satellite continued to drift slowly eastward through the GEO belt. What had seemed like a small problem was about to get much bigger.
On April 30, the issue of possible interference with other satellites was publicly raised for the first time. On May 4, Intelsat announced that Galaxy 15 was too close to another satellite, AMC 11, to attempt any further interventions. Galaxy 15 drifted into AMC 11’s orbital slot around May 23 and is planned to exit around June 7. During this time, it could cause interference with AMC 11’s broadcasts. Over the next few months, Galaxy 15 will continue to drift through the GEO belt and past other satellites, potentially causing more interference along the way.
|As a result, the satellite continued to drift slowly eastward through the GEO belt. What had seemed like a small problem was about to get much bigger.|
Intelsat has announced that they will continue their attempts to regain control or turn off the satellite when the satellite is safely separated from others systems. Given the immense effort Intelsat has already attempted in this regard, it is unlikely that this will succeed. Fortunately, there is a failsafe option. At some point, the momentum wheels used to maintain the satellite’s orientation will saturate and the satellite will lose Earth lock. Once that happens, the satellite will no longer able to point its solar panels at the Sun, will lose electrical power, and will shut down. Even if control cannot be re-established at that point, it will mean Galaxy 15 will no longer be able to interfere with the broadcasts of other satellites. The only question is how long it will be before this happens: Intelsat’s current estimates suggest that the failsafe scenario will occur at some point this summer.
In the meantime, SES, the owner of AMC 11, has announced a plan for minimizing any interference caused by Galaxy 15 as it drifts past. The plan involves moving another satellite, SES-1, into the same orbital box as AMC 11. As Galaxy 15 passes through the area, traffic will be switched to SES-1 and then back to AMC 11 to stay as far away from Galaxy 15 as possible. SES has posted a computer animation of this process on their website. The plan also includes using a very high power antenna owned by Intelsat in Clarksburg, Maryland, to be able to better distinguish between the three satellites in the same box and transmit to the correct one with pinpoint accuracy.
To fully understand the technical issues involved in this event, one needs to take a close look at the space environment. The geosynchronous region is defined by the Inter-Agency Debris Coordination Committee (IADC) as:
a segment of the spherical shell defined by the following:
Lower altitude = geostationary altitude minus 200 km
Upper altitude = geostationary altitude plus 200 km
inclination of ±15 degrees from the Equator
where geostationary altitude is defined as 35,786 km
Within this region is the geostationary belt, defined as a circular orbit 35,786 kilometers in altitude above the Earth with an inclination of zero (meaning it is directly over the Equator). Figure 1 illustrates this region graphically (the geosynchronous region is in blue and the geostationary belt is in green):
Figure 1: IADC protected zones (credit: European Space Agency)
One can imagine the geostationary belt as a giant circular racetrack and the satellites in that orbit as the cars, going around the Earth in the same general direction and altitude. Although satellites in the belt are moving around the Earth at just under 11,300 kilometers per hour (7,000 mph), they appear to an observer on the surface of the Earth to be almost stationary in the sky because GEO spacecraft make one complete orbit in the same time it takes the Earth to rotate once. If a spectator was to stand in the middle of the infield of a circular racetrack and turn in place at the same rate the cars were moving around the track, you would see the same effect—the cars would appear stationary.
Of course nothing in physics is ever this simple in reality, and indeed the situation in geostationary orbit is much more complex. It is virtually impossible to have an exactly circular orbit at exactly the right altitude, which means every object in GEO is in an orbit with some amount of inclination and eccentricity. Thus, instead of being a perfect stationary dot in the sky, satellites in GEO actually drift in a way that appears to trace a racetrack pattern on surface the Earth, with the north and south height of the racetrack corresponding to their inclination.
Satellites that can stay in a relatively fixed position relative to the Earth provide many useful services. The biggest of these is the ability to provide communications, including television broadcasts and voice services. Since the initial concept of a GEO communications satellite was theorized by Arthur C. Clarke in 1945 and the launch of the first commercial communications satellite, Telstar-1, in 1962, it has become a massive global industry. In 2008, the worldwide satellite services industry, much of which utilizes the GEO belt, brought in revenues of $67.3 billion.
Because of this massive demand to place satellites in such a narrow region, there are measures in place to regulate the GEO zone. An international legal framework managed by the International Telecommunication Union (ITU) was put in place to license and distribute satellite frequencies (in 1963) and slots (in 1973) for geostationary orbit. Each state or private entity that wishes to place a satellite in a specific position over the Equator must apply to the ITU for a license and receive permission if they would like their physical position and operating frequency to be protected from interference. The ITU specifies “slots” in GEO by a certain amount of longitude along the Equator. A slot is defined as the separation needed between satellites to prevent them from interfering with others. Over North America these slots are currently two degrees of longitude, which translates to about 1470 kilometers (915 miles) in the GEO belt.
|There are almost twice as many dead and drifting objects in the GEO belt as there are operational payloads.|
Within their assigned slot, a satellite operator usually maintains their satellite within a specific orbital “box”. The size of this box depends on a number of factors, including how precise the satellite’s position needs to be to serve its customers and how accurately the operator can determine and maintain its position. Typically a GEO box is around 0.1 deg of longitude or 70 kilometers (43.5 miles) in length. In certain situations, a satellite operator might be operating multiple satellites within the same slot in what is called a cluster. Clusters allows for a lot more satellites to be packed into the same area, but create significant challenges for keeping the satellites separated.
A satellite operator must periodically perform “station-keeping maneuvers” to stay within their box and slot because there are several natural forces at work in the GEO region called perturbations that change orbits over time. There are three significant perturbations to focus on in GEO. The first major perturbation is the gravitational pull of the Earth. The exact forward velocity of a satellite in orbit around the Earth is determined by the strength of the Earth’s gravitational pull and the altitude of the satellite. The Earth is not a perfect sphere so it follows that its gravitational field, which is a function of its mass, is not the same everywhere around it. In fact, there are two “bulges” along the Equator at approximately 75° East longitude and 105° West longitude, over the Indian subcontinent and North America, respectively. As described earlier, these gravity “troughs” (officially called libration points) pull satellites in geostationary orbit east or west towards whichever point is closest, giving the satellite an apparent east or west drift as viewed from the ground.
The second major perturbation in the GEO orbit is the gravitational pull of the Moon and the Sun. The Moon orbits the Earth at an average altitude of 384,400 kilometers (238,800 miles) and at an inclination of between 18° and 28° with respect to the Equator. This means that the Moon is always either above or below the GEO belt and thus its gravity pulls objects in GEO north or south with respect to the Equator. The Sun, although incredibly massive, is much further away at an average of almost 143 million kilometers (93 million miles) from the Earth, so its gravitational pull has less of an impact on satellite orbits, although it is still important.
The third major perturbation is called solar radiation pressure and it is caused by photons being emitted by the Sun. A photon is the particle that makes up light. It has no mass but carries momentum. The photons being emitted by the Sun striking objects in orbit transfers some of this momentum, and causes a change in the object’s orbit. The amount of change depends on the object’s surface area in relation to its mass. A compact or heavy object will barely be affected by solar radiation pressure. However, for objects with huge solar panels, such as a communications satellite, it can significantly change their orbit over time.
The annual Classification of Geosynchronous Objects, published by the European Space Agency’s Space Debris Office, is the best reference for what sorts of objects are located in GEO and how many there are. The February 2010 report provides the following details:
A total of 1,238 objects known objects are in the GEO region:
- 391 are under some level of control (either in longitude, inclination, or both)
- 594 are in a drift orbit
- 169 have been captured by one of the two libration points
- 11 are uncontrolled with no recent orbital elements available (usually meaning they are lost)
- 66 do not exist in the U.S. military’s public satellite catalog but can be associated to a specific launch
As these numbers illustrate, there are almost twice as many dead and drifting objects in the GEO belt as there are operational payloads. And there are likely to be many more pieces of space debris that have not yet been detected: current space situational awareness (SSA) capabilities can only reliably detect objects to about the size of a basketball at GEO altitudes.
Compounding the problem of space debris are satellites that are left in the GEO belt at the end of their service life. According to the recently adopted United Nations Space Debris Mitigation Guidelines, which are based on the more extensive IADC Guidelines, spacecraft operators are supposed to perform an end-of-life disposal maneuver to remove their satellite from the protected GEO region. This usually involves a series of maneuvers to boost it at least 250 kilometers (155 miles) above the active GEO belt.
Unfortunately, the guidelines don’t resolve problems associated with spacecraft left in the GEO belt during the early years of the space age, and compliance with these guidelines for new spacecraft is still spotty at best. According to the February 2010 Classification of Geosynchronous Objects report, of the 21 GEO spacecraft that reached end-of-life in 2009, only 11 were disposed of properly. Several were moved out of the active belt but not into an orbit high enough to ensure that they do not cause problems in the near future. Three spacecraft, all Russian, appear to have been abandoned in the active belt and are now librating about the 75° East libration point. Four rocket bodies, three Russian and one American, which were used to place payloads in GEO, also orbit within the protected zone.
Figure 2: All known objects in the GEO region. (credit: Analytical Graphics Inc.)
Figure 2 sums up all of this information about active satellites, debris, and drifters to give a picture of what the GEO environment looks like. Active satellites are in green and orange, while space debris is in red. Far from being the simple, straightforward, organized region as it is sometimes portrayed, the GEO environment is in reality a chaotic place. Accurate stationkeeping by all satellite operators is extremely important, and the in-place failure of a satellite like Galaxy 15 makes this problem worse. While it is unlikely that Galaxy 15 will collide with another object in the near future, our current inability to remove it from the active belt means that it will remain in the region essentially forever.
|While it is unlikely that Galaxy 15 will collide with another object in the near future, our current inability to remove it from the active belt means that it will remain in the region essentially forever.|
Over the next several weeks, Galaxy 15 will continue to drift eastward from its original slot at 133° W towards the libration point over North America at 105° W, approximately where the Rocky Mountains are. To picture what happens next, consider a car on top of a hill with a road that goes down into a valley and then back up to another hill. If the car is pushed down the hill, it will go through the bottom and up the other side. If it has enough speed, it will go up and over the other side and escape the trough. But if it does not, then it will stop somewhere on the far hill short of the top and then roll back down, through the bottom, and then back up the first hill. Eventually, after several trips up and down, the car will settle at the bottom of the valley.
The same physics are at work with the libration points. If Galaxy 15’s drift rate is high enough, it will drift through the 105° W libration point and continue across the Atlantic Ocean and perhaps be captured by the 75° E point over India. But it is much more likely that Galaxy 15 will be trapped in the gravitational valley created by the 105° W libration point and oscillate back and forth on either side of it, joining the other 43 pieces of space debris already trapped there, all of which pose a long-term navigation hazard to the GEO satellites located over North America.